FIG. 1 shows a configuration of a general cellular communication system. A plurality of terminals (mobile stations) 103, 104, 105, and 106 make wireless communication with base stations 101 and 102. The base stations 101 and 102 establish communication among the terminals or communication with communication equipment belonging to a fixed network 108, under control of a base-station control station 107. When communication with the communication equipment belonging to the fixed network 108 is established, a part of all the belonging terminals is selected.
There is known means for preferentially selecting a terminal with a higher communicable transmission rate from among the belonging terminals in order to improve throughput of a base station (scheduling), in such a wireless communication system. For example, “Data throughput of CDMA-HDR a high efficiency-high data personal communication wireless system” by A. Jalali, R. Padovani, and R. Pankaj, IEEE VTC-Spring 2000, Tokyo, Japan, May 2000 (Non-patent Document 1) discloses a technique involving causing each of belonging terminals to measure a channel condition and notify it as a maximum receivable transmission rate and determining a terminal with the highest rate of the maximum receivable transmission rate to an average rate as a transmission target, thereby improving the throughput of the base station while fairness among the terminals is secured.
As a method of obtaining a time diversity effect, there is known an HARQ (Hybrid ARQ) method in which retransmission control by an ARQ (Automatic Repeat reQuest) method is performed, and decoding is further performed with the same packet that has already received.
By combining those methods, it is possible to communicate with each terminal with a quality corresponding to the condition of the terminal while securing fairness among the terminals. For example, 3GPP2 C.S0024 Version 4.0“cdma 2000 High Rate Packet Data Air Interface Specification” pp. 9-62 to 9-67 (Non-patent Document 2) discloses a technique in which the maximum number of retransmissions is set for each transmission rate to perform HARQ communication, and scheduling is performed when the retransmission ends (when the maximum number of transmissions has been reached or when notification of completion of decoding is received from the terminal side).
FIG. 2 shows a control sequence of a conventional technique. First, a base station transmits a time-multiplexed signal of transmission data and a pilot signal to a terminal to be a transmission target (201). The terminal calculates an SIR (a signal-to-interference power ratio) from the pilot signal (202) and transmits a DRC (information about the maximum receivable transmission rate under the SIR) to the base station (204). At the same time, the terminal performs error correction decoding of diffused data (203) and transmits a result of error detection (205) to the base station side.
Since the result is transmitted three slots after the reception, the error correction decoding is performed within the three slots. The base station determines whether to perform retransmission or whether to transmit new data based on the error detection result. When transmitting new data, the base station determines a terminal to be a transmission target based on the DRCs of all the terminals (206).
FIG. 3 shows a timing of data transmission at a base station according to the conventional technique. A data decoding result (302) of data transmitted at time 1 (301) is received at time 4. It is judged whether scheduling (303) is necessary or not based on the result, and it is determined whether to retransmit the data 301 transmitted at the time 1 or transmit new data as transmission data (304) at time 5.
Thus, it is necessary to wait for the data decoding result to be returned, and retransmission of data is performed every four slots. Therefore, each of the data at the times 1, 2, 3, and 4 can be independently transmitted, and the base station performs data transmission in four parallels. Description will be made below with a data transmission timing 1 as an example. Correspondence between time t1-1 etc. for each timing and absolute time is as shown in FIG. 3.
FIG. 4 shows a scheduling operation trigger of the conventional technique. The figure shows a case where, at time t1-4, retransmission of first data of a terminal A (hereinafter referred to as “A1”) (401) ends (402); scheduling (403) is performed; the terminal A is selected; and at time t1-5, transmission of data A2 (404) is started. In addition to the case where the data decoding result indicates success of decoding, 402 is also effective in the case where the specified number of retransmissions has been reached.
The scheduling of the conventional technique is performed in two stages of selection based on the priority of held data and selection based on an evaluation index. FIG. 5 shows an example of a case where three terminals A, B, and C belong to a base station, the terminal A having data in each of queues 501, 502, and 503 for storing data with priorities 1, 2, and 3, the terminal B having data only in the queue 506 among queues 504, 505, and 506 for storing data with priorities 1, 2, and 3, the terminal C having data in the queues 508 and 509 among queues 507, 508, and 509 for storing data with priorities 1, 2, and 3. In this case, the terminal A having data in a queue with the highest priority 1 is determined to be a transmission target.
FIG. 6 shows an example of a case where three terminals A, B, and C belong to a base station, the terminal A having data in the queues 502 and 503 among queues 501, 502, and 503 for storing data with priorities 1, 2, and 3, the terminal B having data only in the queue 506 among queues 504, 505, and 506 for storing data with priorities 1, 2, and 3, the terminal C having data in the queues 508 and 509 among queues 507, 508, and 509 for storing data with priorities 1, 2, and 3. In this case, of the terminals A and C having data in a queue with the highest priority 2, the one that has a higher evaluation index to be notified is determined to be a transmission target.
FIG. 7 shows an operation flowchart of the conventional technique. First, the data decoding result of data transmitted last is received (step 701). If the step 701 shows that decoding has been successful or the specified number of retransmissions has been reached, a scheduling operation trigger is made effective (step 702). If the operation trigger for scheduling of the step 702 is effective, then, for all belonging terminals, information about transmission data with the highest priority and channel information to be notified are collected (step 703). If terminals having transmission data exist as a result of the step 703 (step 704), terminals having data with the highest priority are selected as candidates (step 705), and a terminal in which the evaluation index calculated from the channel information collected in the step 703 is the largest is determined to be a transmission target (step 706). Data started to be transmitted is deleted from a transmission queue and is stored in a deletion queue for a predetermined time in consideration of a possibility of retransmission control by a higher layer (step 707).
FIG. 8 shows a configuration of a terminal-side wireless communication device using a conventional method. A signal received by an antenna is converted into a baseband-band signal by a radio frequency circuit (801). The baseband-band signal is inputted to a modulation/demodulation unit (810), and demodulation processing such as detection is performed by a demodulator (802). Then, each coding unit of the signal is decoded by an error correction decoder (804). When decoding is performed by the error correction decoder (804), a demodulation result of the same data received in the past is added (803), and error correction decoding is performed as necessary. As for a result that has been of the error correction by the error correction decoder 804, an error detection unit (805) detects an error thereof, and receiving quality information indicating whether an error exists or not is created. The receiving quality information and the channel information are code-multiplexed with a pilot signal generated by a pilot signal generation unit (806) and a data signal which has been channel-coded by an error correction coder (807), by a multiplexer (808) of the modulation/demodulation unit (810). The multiplexed signal is modulated by a modulator (809) and sent out to a wireless channel via a radio frequency circuit (801).
The signal transmitted from the receiving-side wireless communication device is received by a base-station-side wireless communication device shown in FIG. 9. The base-station-side wireless communication device executes demodulation for all the terminals by a demodulation unit (906). The operations of 801, 802, and 803 are similar to those of the terminal-side wireless communication device. A channel information extraction unit (904) and a data decoding result extraction unit (905) extract the channel information and the data decoding result, respectively. A scheduler trigger creation unit (908) creates an operation trigger based on the data decoding result collected by 906 and the number of retransmissions managed by a number-of-retransmissions management unit (907), and a transmission data determination unit (909) determines transmission data based on the operation trigger and the collected channel information. The transmission data is moved to a data waiting to be deleted management unit (910), channel-coded by an error correction encoder (911), and code-multiplexed with a pilot signal generated by a pilot signal generation unit (912) by a multiplexer (913). The multiplexed signal is modulated by a modulator (914) and sent out to a wireless channel via a radio frequency circuit (901).